Electrocoating apparatus

ABSTRACT

An electrocoating arrangement employing at least two tank baths for improved coating quality, each tank bath containing a resinous water-soluble paint, and a specific tank-article arrangement which provides a high safety factor in that the tank and the article have the same electrical polarity.

United States Patent Koch, H et al.

[ Feb. 8, 1972 [54] ELECTROCOATING APPARATUS [72] Inventors: Robert L.Koch, II; Gilman Tredwell, both of Evansville, Ind.

[73] Assignee: Ashdee Corp., Evansville, Ind.

[22] Filed: Mar. 17, 1965 [21] Appl. No.: 440,410

I [52] U.S.Cl ..204/300,204/l8l,204/301 5s 1 Field of Search ..204/299,300, 18]

[56] References Cited UNITED STATES PATENTS 2,478,322 8/1949 Robinsonetal ..204/181 2,898,279 8/1959 Metcalfe et al ..204/181 3,304,2502/1967 Gilchrist ...204/ l 81 3,355,374 12/1967 Brewer et al ..204/ l 81FOREIGN PATENTS OR APPLICATIONS 970,506 9/l964 Great Britain ..204/181Primary Examiner-Howard S. Williams Attorney-Warren D. Flackbert [57]ABSTRACT An electrocoating arrangement employing at least two tank bathsfor improved coating quality, each tank bath containing a resinouswater-soluble paint, and a specific tank-article arrangement whichprovides a high safety factor in that the tank and the article have thesame electrical polarity.

6 Claims, 2 Drawing Figures PATENTEBFEB 8 I972 3.640.814

Fl E T IN VEN TOR6 ROJERT A. lac/1,1 y Gunny TAPIDIVEIL fforvzeyELECTROCOATING APPARATUS The present invention relates toelectrocoating, and more particularly to a new and novel arrangement andstructure for achieving a more effective and safe process ofanodichydrocoating.

Although relatively in its infancy, electrocoating, sometimes termedanodic-hydrocoating, is a process of applying organic finishes throughthe use of electrical energy. In carrying out the basic process, anarticle to be coated is immersed in an electrolyte, defined as aspecially formulated, water-soluble paint, where electrical current isthen passed through the article and the electrolyte for a given periodof time. Upon application of electrical energy, the resin and pigmentforming part of the aforesaid electrolyte migrate to the article and afilm is irreversibly deposited thereon. Thereafter, and in a typicalprocedure, the article may be rinsed, dried and baked, either directly,or after being overcoated wet on wet with a conventional compatiblepaint.

The process is highly advantageous in the control of film thickness onthe article coated, even to the obtaining of thick films when using apaint which is formulated with a low solid content, and, additionally,results in rapid paint drain-off, good coverage of sharp edges withoutfilm recession, important penetration into welded seams and even intothe interior surfaces of boxed sections, no solvent wash, and reducesany fire hazard by the use of water-soluble material.

In prior arrangements, the process has been characterized by the use ofa single tank, where the implementation thereof was costly, whetheroperated as a batch process or a continuous process. In either the batchprocess or continuous process, of the article to be coated entered thebath rapidly, a costly power supply was required, considering thehigh-current surge at the beginning of the coating process. In the batchprocess, if the article was lowered slowly into the bath, much more timewas involved to accomplish the coating. In a continuous process wherethe article was lowered slowly into the bath, not only way additionalcoating time required, but a much longer tank, necessitating the use ofadditional valuable floor space, as well as additional material in thetank.

Moreover, existing electrocoating arrangements have provided anotherdrawback safetywise, in that the tank is of one electrical polarity,while the article being coated is of an opposite electrical polarity,reflecting possible danger to the operator, and necessitatingcomplicated insulating hangers and commutators, and the use of bus bars,to make electrical contact with the article when it is in the paintbath.

By virtue of the instant invention, the applicants herein have provideda new and novel approach to the aforedescribed electrocoating process,teaching the use of a multitank electrocoating arrangement, and, aswell, a specific tank-article arrangement which results in a high-safetyfactor and economical implementation, in that, in the preferred form ofthe invention, the tank and article have the same electrical polarity.Additionally, and as to be discussed further herebelow, the instantinsulated tank readily lends itself to provision for electrodialysis foramine and pl-I-level control, without the necessity of using threedifferent energy potentials as has typically been the case in the past.

With a multitank arrangement comprising, at least, a first tank havingan approximate constant current of voltage characteristic and a secondtank having an approximate constant voltage or current characteristichigher than that of the first tank, an effective smooth coated film isachievable, where the throw power of the paint component of theelectrolyte and the flow of the paint, which is dependent upon suppliedelectrical energy, is more effectively controlled than has been possibleheretofore. In other words, the instant invention permits a higherelectrical energy level, and, hence, better throw power, in the secondtank, not being limited to the lower nonbubbling" electrical energylevel which is maintained in the known one-tank production arrangementto prevent any rupture of the finish. Moreover, inherent with theinstant arrangement is the need for less article time in the first tank,as

long as a protective film is achieved which will then preclude anybubbling of the coating.

In addition to the aforesaid highly important safety factor resultingfrom having the article being coated and each of the tanks in anymultitank arrangement at the same electrical polarity, the instantinvention provides other advantages with the multitank arrangement,including shorter time for coating, a better control of the film makeupand color, the elimination of staining effect in pastel colors, i.e.,rusting due, for example, to oxidation of ferrous ions upon baking,penetration into even deeper areas, the need for less area or floorspace in any manufacturing activity, and the simplification, andresulting economic savings, of power supply equipment, coupled withbetter flexibility of the latter.

Accordingly, the principal object of the present invention is to providenew and novel apparatus and techniques in electrocoating process forapplying organic finishes.

Another object of the present invention is to provide a multitankarrangement for effecting electrocoating which results in advantages notat all available through previous usage of a single tank operation.

A further object of the present invention is to provide a new and noveltank structure whereby the tank and the article being coated aremaintained at the same electrical polarity.

A still further and more general object of the present invention is toprovide, in an electrocoating process, a high-safety factor for theoperator, an optimum use of equipment and floor space, and importantfinishing results not available heretofore.

Other objects and a better understanding of the present in vention willbecome more apparent from the following description, taken inconjunction with the accompanying drawing, wherein FIG. 1 is a view inelevation, partly fragmentary and partly in vertical section, showing atank structure in accordance with the teachings of the instantinvention; and

FIG. 2 is a plan view of the tank structure of FIG. 1.

For the purposes of promoting an understanding of the principles of theinvention, specific language will be used to describe the embodimentsillustrated in the drawing and described herein. However, it willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications thereof, and such further applications of the principlesof the invention being contemplated as would normally occur to oneskilled in the art to which the invention relates.

In accordance with the present invention, the technique ofelectrocoating is advanced through the provision of a multiple tank batharrangement, instead of the previously utilized single tank bath. Inorder to avoid the high-current surge inherent with the use of a singletank, two tanks are utilized, where the electrolyte in the first tank ispreferably maintained at an approximate constant current or voltage,requiring simplified power equipment than necessary heretofore with thesingle tank arrangement, and where the electrolyte in the second tank ispreferably maintained at an approximate constant current or voltage,being higher in value than that which could be properly employed in asingle tank arrangement,

representing importance to the arrangement by assuring less time for theoverall operation and better results than those achievable heretofore.

While, depending upon the electrolyte in use, the electrical energylevel in the first tank may be lower than that employed in a one-tankarrangement, such electrical energy level is probably equal to that inthe one-tank arrangement to prevent bubbling and any rupturing of thefinish being applied. In any event, standard equipment is typicallyemployed to attain the desired electrical energy levels in the firsttank and in the second tank.

As mentioned hereabove, the use of the instant multitank arrangementrepresents a simplification, both costwise, through equipment and space,and operationally, through control, over the prior known single tankprocedure. In this latter regard, and by way of example, the operatorcan readily avoid a higher than desired voltage in the first tank, wherethe latter is not feasible because of the development of gas bubbles, aswell as burned areas, in the organic finish. On the other hand, becauseof effective control, higher electrical energy can be supplied to thesecond tank than feasible in a single tank arrangement, whereby optimumcoating results through higher power of the paint after initial coatingin the first tank.

In a preferred practice of the invention, the electrolyte used istypically a water base soluble resin, the latter being, by way ofexample, of an acrylic, an epoxy or an alkyd type. Moreover, althoughcommonly termed a paint, actually coating substance might be moreappropriate to define the electrolyte used in the electrocoatingprocess.

Although conventional tank structures may be employed in the instantabove-described multitank arrangement for electrocoating, the applicantsherein have provided a new and novel tank structure, shown in FIGS. 1and 2, which permits, as mentioned hereabove, an important safety factorto the user. In this regard, and representing an important contributionto the art, the applicants tank structure permits the article beingcoated and the tank itself to be maintained at the same electricalpolarity.

In order to accomplish the preceding, a conventional open tank 12 isprovided having upstanding sidewalls and a bottom wall. The innersurface of the tank 12 is typically coated (not shown) withpolyurethane, epoxy, polyesters, or the like, to prevent anyirreversible depositing of coating material or paint thereon. Spacedinwardly around the sidewalls of the tank 12 are a series of electrodes14, where nonconductive members 15 serve as mounting means therefor.

In that the presently practiced electrocoating process requires, becauseof the nature of available paint and in contrast to customary metalplating, the coating of a part which is maintained at a positivepolarity, both the article being coated and the tank 12 are at the sameelectrical polarity, both being grounded, where the electrodes 14 are atan opposite electrical polarity. In other words, any brushing againstthe outside of the tank 12 does not endanger the operator because of thespaced apart relationship between the walls of the tank and theelectrodes 14. In any event, the instant tank teaches an arrangement formaintaining the article being coated and the tank at the same electricalpolarity, where any electrode is, of course, at the opposite electricalpolarity.

The preceding is important because of the high voltages involved inelectrocoating, in contrast, or example, to those employed in usualcommonly known metal plating operations. Moreover, it should be evidentthat the instant tank arrangement does not require any specializedisolating of the article being coated, as the latter is at the sameelectrical polarity as the conveyor on which it moves during the coatingprocess.

In this regard, the figures further illustrate a conveyor 18 incombination with the new and novel above-described tank arrangement. Itmight be again stressed that the use of the instant multitank techniqueaffords an important advantage spacewise in a production area, and alsopermits almost optimum vertical immersing of the article being coated,in contrast to the gradual downwardly and upwardly inclined movementcommon to a single tank approach.

In order to attain optimum film thickness and coating, the electrodes 14forming part of the applicants new and novel tank structure preferablyhave a combined area equal to or greater than the area of the articlebeing coated, and, moreover, instead of utilizing a lined tank, theoverall tank may be made from an electrically insulative material, suchas polyethylene or fiberglass, for example, being electrically neutralin such instance.

Another feature of the instant insulated tank is its adaptability forutilizing the electrodialysis process for pH level and amine control,where previously a complicated arrangement was involved which entailedthree potentials of electrical enerviz, a positive part, a negativeelectrode and a neutral tank. e control of the amine, and, hence, the pHlevel in the tank was accomplished through the use of a bag containingamine absorption material and made from a semipermeable membrane. Withthe invention at hand, an important advantage results in only a screen16, defined as a semipermeable membrane for passage of the amine, butnot the pigment or the like in the paint, need be employed, without anymultielectrical energy problem, adjacent each row of negative electrodesfor the desired results. In other words, the entire electrodialysisprocess has been simplified in accordance with the teachings of theinstant invention where, at the outset, no specialized electricalequipment is required to carry out the process.

From the preceding, it should be understood that the applicants hereinhave provided new and novel contributions to the electrocoating art, inteaching the use of a multitank arrangement, and increased safety andutility in the form of a tank structure per se. As discussed, themultitank arrangement affords many advantages, including, by way ofresume, the need for less floor space for the electrocoating operation,the shorter time required for coating with much more effective finishedresults than possible heretofore, a better control of film makeup andcolor, deeper penetration into recessed areas, a simplification of powersupply equipment, coupled with the flexibility thereof, and theelimination of staining, a problem particularly prevalent with pastelcolors.

It should be further understood that modifications may be made withinthe spirit of the invention. For example, the number of bath tanks maybe increased over the discussed two, different materials used in eachtank, and the electrode area increased for even faster coating. Thus,the preceding description should be considered illustrative and not aslimiting the scope of the following claims.

We claim:

1. A bath tank for electrocoating comprising an upstanding wall and abottom wall, means for holding an article within said bath tank forcoating, and at least one electrode mounted on said upstanding wall ofsaid tank substantially parallel therewith and electrically insulatedtherefrom, and means for maintaining said article holding means and saidtank at the same electrical polarity.

2. The bath tank of claim 1 where said means maintains said articleholding means and said tank at an electrically grounded potential.

3. The bath tank of claim 1 where the inner surface thereof is coatedwith a nonconductive substance.

4. The bath tank of claim 1 where an electrodialysis screen is disposedwithin said tank in spaced-apart relationship with respect to said atleast one electrode.

5. In electrocoating, a first bath tank and a second bath tank eachadapted to contain a resinuous water-soluble paint, conveyor means fortransferring an article to be coated through each of said tanks andserving as one electrode, electrical energy supply means for said firsttank for applying a first coating to said article during a predeterminedfirst time period, and electrical energy supply means for said secondtank for applying a second coating to said article during a secondpredetermined time period of a different duration than said firstpredetermined time period after coating in said first tank, where saidfirst tank, said second tank and said conveyor means are of the sameelectrical polarity.

6. The electrocoating arrangement of claim 5 where said secondpredetermined time period is the same as said first predetermined timeperiod.

2. The bath tank of claim 1 where said means maintains said articleholding means and said tank at an electrically grounded potential. 3.The Bath tank of claim 1 where the inner surface thereof is coated witha nonconductive substance.
 4. The bath tank of claim 1 where anelectrodialysis screen is disposed within said tank in spaced-apartrelationship with respect to said at least one electrode.
 5. Inelectrocoating, a first bath tank and a second bath tank each adapted tocontain a resinuous water-soluble paint, conveyor means for transferringan article to be coated through each of said tanks and serving as oneelectrode, electrical energy supply means for said first tank forapplying a first coating to said article during a predetermined firsttime period, and electrical energy supply means for said second tank forapplying a second coating to said article during a second predeterminedtime period of a different duration than said first predetermined timeperiod after coating in said first tank, where said first tank, saidsecond tank and said conveyor means are of the same electrical polarity.6. The electrocoating arrangement of claim 5 where said secondpredetermined time period is the same as said first predetermined timeperiod.